Since the invention of the transistor in 1947, much effort has been directed towards extension of the device operating range towards higher and higher frequencies.
Conventionally, the cut-off frequency f.sub.T (defined as the frequency at which the current gain .beta., i.e., the absolute value of the parameter h.sub.fe .tbd..differential.J.sub.c /.differential.J.sub.B, is unity) is used as a figure of merit that is indicative of the high frequency capability of a transistor. See for instance, S. M. Sze, "Physics of Semiconductor Devices", 2nd Edition, John Wiley & Sons, 1981, Chapter 3, incorporated herein by reference. It is well known that .beta. at high frequencies decreases at a value of 10 dB/decade.
Another parameter that can be used to characterize the high frequency capabilities of a (typically microwave) transistor is the unilateral (power) gain U. See S. M. Sze, op. cit., pp. 160-165. The frequency at which the unilateral gain is unity is the maximum oscillating frequency f.sub.max, which can, but need not, be larger than f.sub.T. Both f.sub.T and f.sub.max are conventionally determined by extrapolation of the measured roll-off in h.sub.fe and U, respectively.
G. T. Wright, (see, for instance, Solid State Electronics, Vol. 22, p. 399, 1979) proposed extension of active transistor operation of frequencies beyond the conventional cutoff frequency f.sub.T. The proposal involved the utilization of transit time resonances that arise from carrier drift in the collector space charge region. The proposed model suggested for an ideal transistor (i.e., a transistor without any parasitic impedances) the possibility that .vertline.U.vertline. could exceed unity at frequencies above f.sub.max. However, it has now been shown (S. Tiwari, IEEE Electron Device Letter, Vol. 10, No. 12, p. 574, 1989) that the proposed utilization of transit time resonances in a conventional GaAs/AlGaAs HBT would require reductions of each of the base and collector resistances and of the collector capacitance by at least an order of magnitude from state of the art values. Clearly, the proposed mechanism is, at least for the foreseeable future, not likely to be embodied in a practical device. To the best of our knowledge, transit time resonances of the prior art type were not considered with regard to hot electron HBTs. N. Dagli, (Solid State Electronics, Vol. 33 (7), p. 831) proposed a hot electron unipolar transit time transistor.
HBTs with substantially collisionless base transport are known. See, for instance, U.S. Pat. No. 4,829,343. Herein free carrier (not necessarily electron) base transport is considered to be "ballistic" if the mean free path (.LAMBDA.) of the carriers in the base material is 3/8W.sub.B, the base width. As those skilled in the art know, the mean free path can, at least in principle, be determined by transport measurements in a magnetic field.
The cut-off frequency of a prior art ballistic HBT cannot be less than (2.pi..tau..sub.B).sup.-1, where .tau..sub.B is the average base transit time of the minority carriers. Therefore, prior art ballistic HBTs are typically designed to minimize .tau..sub.B. This generally involves maximizing carrier velocity through choice of low effective mass minority carriers (almost invariably resulting in the choice of n-p-n III/V transistors), and through choice of a design that exhibits a relatively large value of the parameter .DELTA., the injection energy. It also typically involves minimization of the base width W.sub.B.
Although HBTs having f.sub.T substantially above 100 GHz have recently been reported (see, for instance, Y. K. Chen, et al. IEEE Electron Dev. Lett., Vol. 10, No. 6, p. 267, 1989), it would clearly be highly desirable to have available transistors that can operate at even higher frequencies. This application discloses such a transistor. The novel device, to be referred to as the coherent transistor (CT), has utility in many fields, e.g., high speed computation or communications.